Myth is a genre of folklore consisting primarily of narratives that play a fundamental role in a society. For scholars, this is totally different from the ordinary sense of the term myth, meaning a belief that is not true, as the veracity of a piece of folklore is entirely irrelevant to determining whether it constitutes a myth.

Myths are often endorsed by religious and secular authorities, and may be natural or supernatural in character. Many societies group their myths, legends, and history together, considering myths and legends to be factual accounts of their remote past. In particular, creation myths take place in a primordial age when the world had not achieved its later form. Origin myths explain how a society's customs, institutions, and taboos were established and sanctified. National myths are narratives about a nation's past that symbolize the nation's values. There is a complex relationship between recital of myths and the enactment of rituals.

The word myth comes from Ancient Greek μῦθος (mȳthos), meaning 'speech', 'narrative', or 'fiction'. In turn, Ancient Greek μυθολογία (mythología 'story', 'legends', or 'story-telling') combines the word mȳthos with the suffix -λογία (-logia 'study'). Accordingly, Plato used mythología as a general term for fiction or story-telling of any kind. This word began was adapted into other European languages in the early 19th century, in a much narrower sense, as a scholarly term for "[a] traditional story, especially one concerning the early history of a people or explaining a natural or social phenomenon, and typically involving supernatural beings or events."

The Greek term mythología was then borrowed into Late Latin, occurring in the title of Latin author Fabius Planciades Fulgentius' 5th-century Mythologiæ to denote what is now referred to as classical mythology—i.e., Greco-Roman etiological stories involving their gods. Fulgentius's Mythologiæ explicitly treated its subject matter as allegories requiring interpretation and not as true events. The Latin term was then adopted in Middle French as mythologie. Whether from French or Latin usage, English adopted the word mythology in the 15th century, initially meaning 'the exposition of a myth or myths', 'the interpretation of fables', or 'a book of such expositions'. The word is first attested in John Lydgate's Troy Book (c. 1425).

From Lydgate until the 17th or 18th century, mythology meant a moral, fable, allegory or a parable, or collection of traditional stories, understood to be false. It came eventually to be applied to similar bodies of traditional stories among other polytheistic cultures around the world. Thus mythology entered the English language before myth. Johnson's Dictionary, for example, has an entry for mythology, but not for myth. Indeed, the Greek loanword mythos (pl. mythoi) and Latinate mythus (pl. mythi) both appeared in English before the first example of myth in 1830.

The main characters in myths are usually non-humans, such as gods, demigods, and other supernatural figures. Others include humans, animals, or combinations in their classification of myth. Stories of everyday humans, although often of leaders of some type, are usually contained in legends, as opposed to myths. Myths are sometimes distinguished from legends in that myths deal with gods, usually have no historical basis, and are set in a world of the remote past, very different from that of the present.

Definitions of myth vary to some extent among scholars, though Finnish folklorist Lauri Honko offers a widely-cited definition:

Myth, a story of the gods, a religious account of the beginning of the world, the creation, fundamental events, the exemplary deeds of the gods as a result of which the world, nature and culture were created together with all parts thereof and given their order, which still obtains. A myth expresses and confirms society's religious values and norms, it provides a pattern of behavior to be imitated, testifies to the efficacy of ritual with its practical ends and establishes the sanctity of cult.